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Reduced muscle strength is suggested as a risk factor for knee osteoarthritis (OA). Meniscectomy patients have an increased risk of developing knee OA. The aim of this study was to identify reductions in different aspects of muscle strength as well as objectively measured and self-reported lower extremity function in middle-aged patients who had undergone a meniscectomy compared with controls.
Thirty-one patients who had undergone surgery in 2006 and 2007 (mean ± SD age 46 ± 6 years, mean ± SD body mass index [BMI] 26 ± 4 kg/m2, and mean ± SD postsurgery 21 ± 6 months) and 31 population-based controls (mean ± SD age 46 ± 6 years and mean ± SD BMI 26 ± 4 kg/m2) were examined for maximal muscle strength and rapid force capacity, distance achieved during the one-leg hop test, and the maximum number of knee bends performed in 30 seconds. The Knee Injury and Osteoarthritis Outcome Score (KOOS) was used to determine self-reported outcomes.
No differences were detected in any muscle strength variables between the operated and nonoperated leg (mean ± SD quadriceps maximum voluntary contraction of 2.80 ± 0.10 for the operated leg and 2.88 ± 0.10 for the nonoperated leg), between patients and controls (mean ± SD torque of 2.70 ± 0.09 Nm × kg-1 for the controls; P = 0.26 for main effect leg), or in objectively measured function (P ≥ 0.27). Patients reported 10–26 points worse KOOS scores in all 5 subscales (P < 0.001).
Thigh muscle strength is not impaired in middle-aged adults 2 years after resection of a degenerative tear. Our findings indicate that factors other than muscle strength are responsible for the perceived functional limitations and suggest that training to improve strength alone may not be sufficient to improve self-reported function in patients at high risk of knee OA.
Reduced muscle strength is suggested as a risk factor for knee osteoarthritis (OA) (1–4). Meniscectomy patients constitute one subgroup at high risk of developing knee OA (5, 6), with approximately one-half of the patients showing radiographic signs of knee OA 10–15 years after undergoing a meniscectomy (6–8). Meniscus tears, referred to as symptomatic degenerative tears, have been suggested to be associated with incipient OA and/or represent early-stage OA in the middle-aged population (9, 10). Therefore, patients undergoing surgery for symptomatic degenerative tears constitute a model to study the state prior to knee OA.
In meniscectomy patients, conflicting results have been reported on the rehabilitation of muscle strength and functional performance in the operated leg compared with the nonoperated leg (11–17). In one study, where a control group was included, a reduction in bilateral quadriceps maximal voluntary contraction (MVC) and voluntary activation was observed in meniscectomy patients (18), but no differences were seen between the operated and nonoperated leg. This finding suggests bilateral muscular deficits and emphasizes the need for a control group if the magnitude and nature of lower extremity muscle strength deficiencies in these patients are to be identified and examined.
Testing maximal isometric muscle strength might not adequately describe all aspects of lower extremity muscle function and changes in functional performance compared with healthy controls. For instance, reduced levels of eccentric thigh muscle strength might reduce the ability to absorb impacts and thereby increase knee joint loading (19). Furthermore, impaired capacity for rapid force exertion (i.e., a right shift in the torque time curve indicating a reduced rate of force development [RFD]) is thought to be important for functional performance (20). Reduced rapid force capacity would decrease the percentage of force produced in the early phase of muscle contraction and thereby reduce the ability to react on sudden perturbations, potentially representing a functional deficiency in postural control and other types of reactive motor tasks. Understanding in detail these muscle strength deficiencies and their relationship to self-reported function in patients at high risk of knee OA would be important in clinical decision making and in developing future knee OA prevention regimens.
The aim of the present study was to identify reductions in various aspects of muscle function for the knee extensor and flexor muscles (i.e., maximal concentric, eccentric, and isometric muscle strength, as well as rapid force capacity) together with deficiencies in objectively measured and self-reported physical function in middle-aged meniscectomy patients compared with population-based controls. It was hypothesized that meniscectomy patients would show a bilateral deficiency in muscle strength and objectively measured function compared with controls.
PATIENTS AND METHODS
Patients ages 35–55 years at the time of surgery, who had undergone surgery in 2006 and 2007 for a medial meniscal tear in the posterior half of the meniscus, were identified through the surgical code system from 2 different hospitals. The age criteria were set to include a majority of patients with degenerative meniscal tears but without knee OA. Patients were excluded if they were misclassified by the surgical code system, or if they had a previous knee ligament injury, severe cartilage changes defined as deep clefts or visible bone at the time of the meniscectomy, or self-reported comorbidities limiting participation in the study. A modified version of the Self-Administered Comorbidity Questionnaire, developed by Sangha et al (21), was used to identify comorbidities. After their meniscectomy, patients were given a leaflet with standard rehabilitation exercises, which they were encouraged to perform at home. Information on compliance with the exercise recommendation was not collected.
Age- and sex-matched controls were identified through the Danish Civil Registration System. An invitation was sent to a total of 600 people living in the same geographic area as the meniscectomy patients. Those who accepted the invitation received the modified version of the Self-Administered Comorbidity Questionnaire (21). Subjects were excluded if they had had a previous knee ligament injury, knee surgery, or self-reported comorbidities limiting participation in the study. Eligible controls were stratified into 4 groups: men ages 35–45 years, men ages 46–55 years, women ages 35–45 years, and women ages 46–55 years. For every patient who was included in the study, a control was randomly selected from the appropriate age group by use of a random number generator. All testing was conducted in the period from August to November 2008.
Testing was done on 2 separate occasions. On the first occasion the informed consent form was signed and the subjects answered the Short Form 36 (SF-36) health survey and the Knee Injury and Osteoarthritis Outcome Score (KOOS), both self-administered questionnaires. Then body height and weight were measured, and the patient's fat free mass was estimated by a conventional bioimpedance leg-to-leg method (TANITA TBF-305, Tanita). The subjects were then tested for the maximum number of knee bends they could perform within 30 seconds and for the distance they could cover in the one-leg hop test (22). All muscle strength tests were carried out in an isokinetic dynamometer (Kinetic Communicator 500H, Chattecx) on the second occasion ∼7 days later. Also on the second occasion, subjects were asked to report the amount of vigorous and moderate physical activity in which they had engaged during work and leisure time over the previous 7 days. Subjects then performed a warm-up activity consisting of a submaximal bicycle exercise test, during which maximal oxygen consumption (VO2max) was estimated using Åstrands nomogram (23). Finally, the muscle strength tests were conducted. The order of test leg was randomized for both groups on both testing occasions (i.e., operated/nonoperated in the patients and left/right in the controls). The study was approved by the ethics committee of the Region of Southern Denmark.
Muscle strength testing.
Placement of subjects in the isokinetic dynamometer was conducted as previously described (24, 25). The dynamometer force and position signals were recorded by a personal computer at a 1,000 Hz sampling rate during isometric tests and a 100 Hz rate during dynamic tests, and was filtered by a fourth-order zero-lag Butterworth low-pass filter at 15 Hz cutoff frequency. To correct for the effect of gravity on the measured joint torques, the passive mass of the lower leg was measured in the dynamometer at a knee joint angle of 45° (26). In addition to the submaximal bicycle warm-up, subjects performed a further warm-up and preconditioning exercise in the dynamometer, which consisted of 4 concentric and 3 eccentric contractions, gradually increasing force. Subsequently, maximal concentric, eccentric, and isometric muscle strength were measured unilaterally in the knee extensors and knee flexors in both legs of all participants. Knee joint angular velocity during dynamic testing was set to 30°/second with a knee joint range of motion from 90° to 20° (0° = full knee extension). Successive trials at each contraction mode were conducted until the subject was unable to further increase peak torque (4–6 trials were typically conducted).
Isometric MVC was measured at a 70° knee joint angle (best of 3 trials) during knee extension and flexion, respectively. The average slope (▵ torque/▵ time) at time points 0–30, 50, 100, and 200 msec of the torque time curve was calculated to evaluate the capacity for rapid force production. These time intervals have previously been used to assess RFD in the initial phase of muscle contraction in healthy patients (25, 27) and in surgical patients (20). The onset of contraction was defined as the instant where force increased by 2% of peak torque above the resting baseline level. Visual feedback of the dynamometer torque output was provided to the subjects on a computer screen after each trial (28). The reliability and validity of the KinCom dynamometer have previously been verified in detail (29).
The KOOS was used to assess function during activities of daily living (ADL) and during sport and recreation function (Sport/Rec). The KOOS also assesses pain, other symptoms, and knee-related quality of life (30). A normalized score is calculated for each subscale (0 indicating extreme symptoms and 100 indicating no symptoms). The KOOS score has been validated for meniscectomy patients and has shown high test–retest reproducibility (30, 31).
The SF-36 was used to assess general physical function (32). The SF-36 consists of 8 subscales: physical function, role physical, bodily pain, general health, vitality, social function, role emotional, and mental health. The SF-36 is self-explanatory, takes ∼10 minutes to complete, and is scored from 0–100 (0 = extreme problems and 100 = no problems). The Acute Danish version of the SF-36 was used (33, 34).
Physical activity level was reported as the amount (in hours and minutes) of vigorous and moderate physical activity undertaken during the previous 7 days and reported separately for work and leisure time. Each period of physical activity had to last for at least 10 minutes at a time. Vigorous physical activity was defined as activity that makes one breathe harder than normal, and moderate physical activity was defined as activity that makes one breathe somewhat harder than normal.
Functional capacity tests.
The one-leg hop test (35) has been shown to be valid and reliable in meniscectomy patients (22). The subject stands on one foot with the hands on the back and is asked to jump as far as possible and land steadily on the same foot. The subject has to be able to land and stand long enough for the examiner to measure the length of the jump. At least 3 trials with 60-second rest periods between each attempt (or until the subject made no further progress) were conducted, and the longest jump was recorded.
The maximum number of knee bends performed in 30 seconds (16) has also been shown to be a valid and reliable test in meniscectomy patients (22). The subject's long axis of the foot was aligned with a straight line and the toes placed on a perpendicular line; light fingertip support was provided to the subject by the examiner to aid balance. The subject was asked to flex the knee while standing on one leg, without bending forward at the hip, until the line along the toes was no longer visible to the subject (∼30° knee flexion). The maximum number of knee bends performed in 30 seconds was recorded.
Becker et al have previously reported a difference of ∼20% in quadriceps MVC between meniscectomy patients and controls (18). To detect a similar difference in quadriceps MVC (80% power with a significance level of 0.05), our sample size calculation indicated a need for 25 individuals in each group. Student's unpaired t-test and the Mann-Whitney test were used to compare subject characteristics between patients and controls, as appropriate. To evaluate the differences in strength variables and functional performance of the patient and control legs, a mixed linear model was used with “subject” as random effect and “leg” (i.e., operated, nonoperated, and control legs), “sex,” and “age” as fixed effects (i.e., applying age as a continuous covariate). Differences among legs in the torque time curve pattern (and therefore RFD) were also assessed by using a mixed linear model with the combination of “subject” and “side” (i.e., repeated nested measurements on each leg of the subjects) as random effects and “leg” (i.e., operated, nonoperated, and control legs), “sex,” “age,” and “time point” (i.e., 0–30, 50, 100, and 200 msec) as fixed effects. Age and sex were introduced in the models to adjust for potential confounding, as anticipated in classic epidemiology. The Mann-Whitney test was used to compare self-reported outcomes (i.e., KOOS and SF-36 scores) in patients and controls. Correlation analyses were performed by calculation of Spearman's rho to assess the relationship between Sport/Rec and concentric, eccentric, and isometric peak torque. Stata software, version 10.1 (StataCorp) was used for all statistical analyses, with the prespecified level of significance equal to 0.05.
A detailed overview of the recruitment flow is shown in Figure 1. Thirty-one patients (10 women) and 31 controls (12 women) were ultimately examined. Due to a slow recruitment process of patients in the beginning of the study, the first 7 controls were included before patients were included, which is the reason for the discrepancy between the number of women, 10 and 12, respectively, in the patient and control groups. Therefore, the study is matched on a group level instead of on a case level.
Characteristics of patients and controls are shown in Table 1. No differences were observed between patients and controls except for physical activity at work, where patients were more active than controls. At the first test session, 5 patients reported knee injuries (which had not been reported previously) in the contralateral knee, 3 patients underwent a meniscectomy, 1 patient had a deficient anterior cruciate ligament (ACL), and 1 patient had an ACL reconstruction and meniscectomy. These patients were not excluded since our a priori hypothesis was a bilateral strength deficit between patients and controls. Furthermore, their results in the strength and functional tests were within 2 SDs of the mean of the nonoperated leg. All other patients had a healthy control leg.
Values are the mean ± SD unless otherwise indicated. N/A = not applicable; VO2max = maximal VO2.
N = 30.
Distribution of physical activity was skewed and is shown as the median (25th, 75th percentiles).
Significantly different than patients (P ≤ 0.05 by Mann-Whitney test).
For which subjects were either treated and/or no limitation imposed for study participation. Musculoskeletal comorbidities reported other than knee: joint problems and back pain. General comorbidities reported: high blood pressure, diabetes mellitus, ulcer, heart problems, and depression.
Thigh muscle strength and objectively measured functional capacity.
Overall, no differences were observed between the operated and nonoperated legs of the patients or when compared with controls in any of the strength variables (Table 2). No differences were observed in rapid force capacity (i.e., no right shift in the torque time curve and therefore no reduction in RFD) during maximal isometric knee extension (P = 0.18) (Figure 2) and knee flexion (P = 0.71) (data not shown). Furthermore, no significant differences were observed in the one-leg hop test or the maximum number of knee bends in 30 seconds test (Table 2).
Table 2. Isokinetic muscle strength and functional performance tests*
Operated leg (n = 31)
Nonoperated leg (n = 31)
Controls (n = 31; 62 legs)
Values are the mean ± SE, adjusted for age and sex. Mixed linear model. P indicates main effect of “leg.” MVC = maximum voluntary contraction.
Isokinetic test patients, n = 29. One patient failed to meet for the second test, and 1 patient did not complete the test due to severe pain after the warm-up procedure.
Patients reported worse knee function than controls in the KOOS subscales for ADL (P ≤ 0.001) and for Sport/Rec (P ≤ 0.001), as well as worse general physical function scores for the SF-36 subscale (P ≤ 0.001). Additionally, patients reported more knee pain, bodily pain, other knee symptoms, and worse knee-related quality of life scores than controls (Figures 3 and 4).
Relationships between self-reported function and objectively measured function and maximal muscle strength.
Weak and nonsignificant correlations were observed between Sport/Rec and the various muscle strength variables, i.e., concentric and eccentric peak torque and isometric MVC (rs = 0.07–0.23, P ≥ 0.22).
To our knowledge, the present study is the first to examine detailed aspects of muscle strength together with objectively measured and self-reported functional capacity in meniscectomy patients compared with age- and sex-matched controls. Despite that, patients in accordance with previous studies reported impaired function, elevated pain, and more symptoms than controls (17, 36, 37). The hypothesized bilateral reductions in various muscle strength variables and functional tests between patients and controls were not observed. Likewise, no differences were observed between the operated and nonoperated leg in the patients, even though this finding could be masked by the patients with injuries to the contralateral knee.
Previous knee injury and reduced muscle strength are considered risk factors for knee OA (1, 2, 38). This study investigated different aspects of muscle strength and functional capacity in meniscectomy patients who were considered to represent a “pre-OA” state (10, 18). Some previous studies report quadriceps strength deficiencies (11, 13), whereas others observe no difference in muscle function (12, 14, 15) when comparing the operated with the nonoperated leg in meniscectomy patients. In 2 recent studies on strength deficiencies 4 years after meniscectomy in patients similar to those in the present study, impaired knee extensor strength and functional capacity were observed in the operated versus the nonoperated leg (17) and bilaterally compared with controls (18). The present study was conducted to combine the strength of these 2 studies by elucidating detailed aspects of muscle strength and functional performance impairments in meniscectomy patients compared with age- and sex-matched controls, and relating these aspects to self-reported knee function. Unexpectedly, however, the initial hypothesis of bilateral strength deficiencies between patients and controls could not be verified. Furthermore, no differences were detected between the operated and nonoperated legs of the meniscectomy patients in maximal muscle strength and functional capacity tests (Table 2 and Figure 2).
The discrepancies between the muscle strength data obtained in the present study and previous studies (17, 18) might be due to differences in patient selection, selection of controls, and/or methodologic differences. In fact, patient recruitment per se could be suspected to be a major reason for the divergent results on muscle strength reported in meniscectomy patients. The recruitment process is often sparsely reported in previous studies. Patient recruitment from a clinical setting or advertising for patients may introduce selection bias toward patients with more symptoms. In the present study, great care was taken during the recruitment process to ensure that patients were representative of a population who had undergone a meniscectomy for a degenerative tear. Furthermore, the meniscectomy patients in the present study were very similar to controls except for the self-reported knee problems (Table 1 and Figure 3). In the study by Becker et al (18), none of the patients participated in sports activities and no information on participation in sports was given for the controls. Therefore, the reported difference between the patients and controls (18) might be influenced by a higher level of physical activity in the control group. In the current study, patients self-reported more moderate and vigorous physical activity at work than controls, whereas physical activity during leisure time was equal between the patient and control groups (Table 1). However, general muscle strength (grip strength) and general fitness (VO2max) were equal in patients and controls supporting similar physical activity levels (Table 1). In the present study, controls were not excluded if they had minor self-reported comorbidities that were similar to those of the patients, since this was to be expected in a population of middle-aged individuals. No information was given on comorbid conditions in the study by Becker et al (18); therefore, it is not possible to know if different levels of comorbidities between the studies could help explain the inconsistent results.
In the current study, patients were examined ∼21 months after their meniscectomy, whereas in the studies by Becker et al (18) and Ericsson et al (17), patients were examined ∼48 months postmeniscectomy. Therefore, it is possible that the patients in our study showed no reduction in muscle strength, since the potential development of knee OA may not have progressed as dramatically due to the shorter postsurgery time interval. However, the ages of the participants in the present study and the 2 previous studies are similar, with a mean age range of 42.8–46.0 years (17, 18).
Changed muscle activation patterns and increased agonist–antagonist coactivation have been observed in knee OA patients compared with healthy age-matched controls (39–42). Hence, impaired neuromuscular function might precede knee OA and also precede muscle weakness. It seems from the present results that when patients performed isolated unilateral single-joint motor tasks in an isokinetic dynamometer, a test with very few degrees of freedom, no difference in performance was observed compared with controls. However, in functional tests that rely more on sensory input and postural (sensory motor) control, there was a borderline clinically relevant difference of 10% between patients and controls. The study was, however, not statistically powered to detect this difference, since sample size estimation was done on quadriceps MVC. Consequently, future studies should examine whether meniscectomy patients have altered neuromuscular activity when performing functional tasks/daily activities or whether sensory input is impaired. Another implication of these findings is that it might be useful if the training of symptomatic patients at risk of OA included training to increase neuromuscular control instead of only muscle strengthening.
A weak and nonsignificant relationship was observed between Sport/Rec and muscle strength variables, indicating the tests were measuring different constructs. Factors other than maximal isolated leg muscle strength could potentially also affect self-reported knee function. Factors like pain, mental health, general fitness (VO2max), and physical activity level are likely candidates to influence self-reported Sport/Rec function. Furthermore, self-reported knee function has been shown to be related to general subject characteristics like age, sex, and body mass index (BMI) (43, 44). A backward stepwise multiple regression analysis with these variables was conducted to explore other potential determinants of self-reported Sport/Rec function. Meniscectomy, poor mental health, and a higher BMI were found to be associated with worse KOOS Sport/Rec function (multiple regression coefficient r2 = 0.52, P < 0.001). Having had a meniscectomy was the strongest predictor (r2 = 0.35) of a worse Sport/Rec function score in the model. Other factors like fear of pain and re-injury or self-efficacy could also affect self-reported function, but we have not evaluated these aspects.
There are some limitations to the present study. The results cannot be generalized to all meniscectomy patients since we followed strict inclusion/exclusion criteria to include a majority of patients with degenerative meniscal tears. Therefore, the results may only apply to middle-aged patients with degenerative tears and not to younger patients who have experienced traumatic tears. Furthermore, only approximately one-third of the patients who received an invitation participated in the study. We have no information on patients that did not participate in the study and direction of any bias due to this can only be speculative. However, the patients self-reported the same amount of pain, symptoms, and functional limitations as previously seen in other studies with similar patients (17, 37). Controls from the same geographic region as the patients were recruited through the Danish Civil Registration System. Nevertheless, one cannot be sure that the controls are representative of the general population from which they were recruited, even though care was taken during the recruitment process to exclude only controls with knee problems related to the research question and to include controls with minor comorbidities common in the general population to this age group.
Study groups (i.e., patients and controls) were not exactly matched on sex (i.e., 10 and 12 women in the patient and control groups, respectively), which could potentially have influenced the results. However, all variables concerning strength and functional capacity had been adjusted for age and sex. Five patients had an injury to the contralateral knee. These patients were not excluded since our a priori hypothesis was a bilateral strength deficit between patients and controls. Furthermore, their results in the strength and functional tests were within 2 SDs of the mean of the nonoperated leg. However, it cannot be ruled out that these patients could mask differences between the operated and nonoperated leg. Excluding these 5 patients from the analysis resulted in a difference range of 6–8% in isometric quadriceps and hamstrings strength between the patients' operated and nonoperated legs, respectively. Differences less than 10% are often not considered clinically relevant (see supplementary Table 1, available in the online version of this article at http://www3.interscience.wiley.com/journal/77005015/home).
In conclusion, thigh muscle strength is not impaired in middle-aged adults 2 years after resection of a degenerative meniscal tear. Our findings indicate that factors other than muscle strength are responsible for the perceived functional limitations and suggest that training to improve strength alone may not be sufficient to improve self-reported function in patients at high risk of knee OA.
All authors were involved in drafting the article or revising it critically for important intellectual content, and all authors approved the final version to be submitted for publication. Mr. Thorlund had full access to all of the data in the study and takes responsibility for the integrity of the data and the accuracy of the data analysis.
Study conception and design. Thorlund, Roos.
Acquisition of data. Thorlund.
Analysis and interpretation of data. Thorlund, Aagaard, Roos.